U.S. patent application number 09/681141 was filed with the patent office on 2002-07-25 for method and apparatus for evaluation of insulation in variable speed motors.
Invention is credited to Feist, Thomas Paul, Krahn, John Raymond.
Application Number | 20020097065 09/681141 |
Document ID | / |
Family ID | 24734008 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020097065 |
Kind Code |
A1 |
Krahn, John Raymond ; et
al. |
July 25, 2002 |
Method and apparatus for evaluation of insulation in variable speed
motors
Abstract
A signal obtained from the drive input of a motor or an
insulated winding is applied to a sensor that strongly rejects high
frequency drive signal components yet passes discharge signal
components. This allows discharge signal components to be detected
in the presence of harmonics of the drive signal. The voltage of
the drive signal may be varied so that the lowest drive signal
voltage producing a discharge event may be determined. That voltage
is the motor or winding's corona inception voltage.
Inventors: |
Krahn, John Raymond;
(Schenectady, NY) ; Feist, Thomas Paul; (Clifton
Park, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
CRD PATENT DOCKET ROOM 4A59
P O BOX 8
BUILDING K 1 SALAMONE
SCHENECTADY
NY
12301
US
|
Family ID: |
24734008 |
Appl. No.: |
09/681141 |
Filed: |
January 25, 2001 |
Current U.S.
Class: |
324/765.01 |
Current CPC
Class: |
G01R 31/343
20130101 |
Class at
Publication: |
324/772 |
International
Class: |
G01R 031/34 |
Claims
1. A method for evaluating insulation defects in a variable speed
motor, comprising: determining a frequency spectrum of a drive
signal applied to a variable speed motor; selecting a sensor having
a transfer function characterized by a cutoff at a frequency within
an overlap of the frequency spectrum of the drive signal and a
frequency spectrum of a discharge event, and having a rejection for
reducing components of said drive signal to below a predetermined
detection threshold for a discharge event; applying to the sensor a
signal S obtained from a drive input of said motor receiving said
drive signal; varying the voltage of the drive signal applied to
said motor; and determining a lowest voltage of the drive signal
associated with inception of a discharge event represented in
output of the sensor.
2. The method claimed in claim 1, further comprising determining a
polarity of the drive signal associated with inception of a
discharge event.
3. The method claimed in claim 1, further comprising determining a
magnitude of a detected discharge event.
4. The method claimed in claim 1, wherein said sensor has a cutoff
frequency in a range of approximately 5 MHz to 20 MHz, and a
rejection of at least 60 dB within one decade from the cutoff
frequency.
5. The method claimed in claim 1, wherein varying the voltage of
the drive signal comprises raising the voltage of the drive signal,
and wherein determining a lowest voltage comprises determining a
voltage associated with a first detection of a discharge event.
6. The method claimed in claim 1, wherein said drive signal is
comprised of one of a single pulse and a series of pulses that
approximates a sinusoidal waveform.
7. The method claimed in claim 1, wherein the signal S is obtained
from a drive input of the motor by a high frequency current
transformer of the sensor.
8. A system for on line analysis of a variable speed motor
comprising: a sensor receiving a signal obtained from a drive
signal input of a variable speed motor driven by a pulse drive
signal, the sensor having a transfer function for rejecting
components in a frequency band of the drive signal that overlaps a
frequency band of a signal produced by a discharge event, and for
passing components of a signal produced by a discharge event; a
detector receiving output of the sensor for detecting components of
a signal produced by a discharge event; and an analyzer for
determining a voltage of the drive signal at inception of a
detected discharge event.
9. The system claimed in claim 8, the analyzer further determining
a polarity of the drive signal associated with inception of a
discharge event.
10. The system claimed in claim 8, the analyzer further determining
a magnitude of a detected discharge event.
11. The system claimed in claim 8, wherein the sensor has a cutoff
frequency in a range of approximately 5 MHz to 20 MHz, and a
rejection of at least 60 dB within one decade from the cutoff
frequency.
12. The system claimed in claim 8, wherein the sensor comprises a
high frequency current transformer coupled to the drive signal
input of the variable speed motor.
13. The system claimed in claim 8, wherein the drive signal is
variable in voltage.
14. The system claimed in claim 13, further comprising means for
varying the voltage of the drive signal.
15. The system claimed in claim 8, wherein said drive signal is
comprised of a series of square pulses that approximate a
sinusoidal waveform.
16. An apparatus for determining the corona inception voltage of an
insulated winding, comprising: a transformer coupling a signal from
an input of a winding driven by a high voltage drive signal
comprised of a series of square pulses that approximate a
sinusoidal waveform; a filter, the transformer and the filter
operating together to reject components of the signal passed by the
means for coupling such that components of the signal that overlap
components of signals produced by discharge events are suppressed
to below a detection threshold of said components of signals
produced by said discharge events; and means for producing output
indicative of a discharge event detected in an output signal of the
filter.
17. The apparatus claimed in claim 16, wherein the transformer and
filter provide a cutoff frequency in a range of approximately 5 MHz
to 20 MHz, and a rejection of at least 60 dB within one decade from
the cutoff frequency.
18. The apparatus claimed in claim 16, wherein the filter comprises
a high pass filter having eight or more poles.
Description
BACKGROUND OF INVENTION
[0001] The invention pertains to variable speed motors, and
particularly to on line evaluation of the health of insulation in a
variable speed motor.
[0002] Reliability is critical to the operation of industrial
systems in which motors are employed. According to several recent
studies of industrial motors, nearly 40% of all motor failures
occur as the result of damage to the electrical insulation in the
motor winding. Damage or excessive electrical stress to insulation
may ultimately lead to catastrophic failure of the motor
winding.
[0003] Insulation health is closely related to electrical discharge
activity within the electrical insulation system of the motor.
Electrical discharges are generally produced by differences between
the potential of adjacent windings, adjacent turns, or between a
winding and a metallic component such as the stator core.
Turn-to-turn electrical discharge is caused by the turn-to-turn
electrical stress and exacerbated by the fast rise times of new
drive technology. The resulting discharge, sometimes referred to as
a partial discharge event or simply discharge event, is a current
spike on the order of 10-100 micro Amps, and having a duration of
approximately 10-100 ns. Due to the transfer function of the motor
winding and the propagation path of the discharge signal to a
sensor, the discharge signal may be corrupted. This results in a
broad frequency spectrum for the discharge signals generally
beginning in the range of 100 kHz and extending to approximately
200 MHz or more.
[0004] Insulation systems are generally classified into either
corona-resistant or non-corona-resistant. Non-corona resistant
insulation systems must operate without discharge activity or
premature failure will occur. Corona-resistant systems may operate
for long periods with electrical discharge before failure occurs.
Unless specified otherwise, all references herein are to
non-corona-resistant insulation systems.
[0005] The voltage at which discharge occurs is the corona
inception voltage (CIV). The higher the CIV, the stronger the
winding insulation, and therefore the longer the motor life. The
discharge event corresponds to a process of insulator degradation.
Degradation is a complex process involving a cascade of electrons
at the discharge site. This cascade accelerates in the electric
field and impacts one side of the discharge site. The electrons
ionize and break down the material, preferentially attacking
organic species (such as the polymeric binder of the insulation
system). The ions then react with available species (such as
oxygen, water, etc.) and form acids. These acids then further
degrade the insulation system. Furthermore, the impact of the
electrons on the surface also causes localized heating, resulting
in thermal degradation. Thus, measuring and monitoring the
electrical discharge activity, and particularly the corona
inception voltage, provides a means of assessing the health of the
machine and allows one to make an estimate of risk to continued use
or expected life.
[0006] For purposes of the present disclosure, motors are divided
into two categories based on the type of drive signal that they
employ. A first type of motor, often referred to as a synchronous
motor, is driven by a simple 50 or 60 Hz sine wave. This drive
signal is relatively pure, and therefore has few harmonics. As a
result, the frequency spectrum of the drive signal, including
harmonics, is relatively confined, and does not overlap with the
signals produced by discharge events, thus, the overlapping
harmonics of the drive signal can be filtered out using a very
simple capacitor filter.
[0007] The second type of motor, which will be referred to herein
as a variable speed motor (also known as an inverter drive motor or
an adjustable speed drive motor), is driven by a pulse generator
that is controlled to produce a simulated sine wave consisting of a
series of square pulses. In other words, the output waveform is
composed of a series of discrete pulses that are selected in their
amplitude and pulse width to effect a sinusoidal waveform. While
such a drive waveform powers the motor in the effectively same
manner as a true sine wave, the individual square wave pulses
contribute harmonics that far exceed those of a synchronous drive
signal in both amplitude and breadth of spectrum. The frequency
spectrum of the drive signal may exceed 5 MHz and therefore largely
overlaps the partial discharge event frequency spectrum.
Furthermore, since under operating conditions the drive waveform is
typically on the order of 100 kV and 100 A, the drive harmonics
will exceed the discharge event signal by 50 dB or more. These
harmonics would saturate the typical amplifier/detection circuits
commonly used for discharge detection in prior art synchronous
motor applications, thereby rendering it impossible to monitor the
health of the insulation system while the motor is on line. Thus
the prior art methods for detecting discharge in synchronous motors
are ineffective when applied to variable speed motors.
SUMMARY OF INVENTION
[0008] Motors may be tested in one of two modes: (1) off-line and
(2) on-line. Off-line testing involves injecting a representative
fast-rise time pulse(s) into the winding either at regularly
increasing voltage levels until discharge activity is detected, or
at a predetermined voltage level wherein a measure of the total or
peak discharges are measured. In off-line testing, the motor is not
performing work. On-line testing involves merely detecting the
presence of discharges while the motor is operated, performing
work, using the drive. If a customized drive is available, allowing
variable voltage control, then inception voltage may be
determined.
[0009] Accordingly, embodiments of the invention enable detection
of discharge events in variable speed motors under actual operating
conditions, i.e. while being driven by a variable speed drive
waveform of typical operating voltage and amplitude. Further
embodiments of the invention is to provide a method and an
apparatus for detecting signals produced by discharge events in the
presence of harmonics of a variable speed drive signal, or a fast
rise time waveform. Still further embodiments of the invention is
to provide a method and an apparatus for detecting the corona
inception voltage for a variable speed drive motor by monitoring
for discharge activity in the presence of harmonics of the variable
speed drive signal or a fast-rise time waveform.
[0010] Embodiments of the invention may apply a signal obtained
from the drive input of a motor to a sensor that strongly rejects
high frequency drive signal components yet passes discharge signal
components. The sensor thus allows discharge signal components to
be detected in the presence of harmonics of the variable speed
drive signal. The aforesaid sensor may be applied while varying the
voltage of the drive signal and determining the lowest drive signal
voltage at which a discharge event is detected, thereby identifying
the motor's corona inception voltage.
[0011] Further embodiments of the invention may determine discharge
magnitude and duration relative to the input drive signal
(phase-resolved analysis), and determine an electrical discharge
trend that can be used to plan scheduled outage/maintenance or to
provide an estimate of risk to continued operation, or that can be
used for design quality assurance or manufacturing quality
assurance and control.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIGS. 1 and 2 illustrate systems for off line and on line
analysis of a variable speed motor, respectively;
[0013] FIG. 3 illustrates the characteristics of an exemplary
sensor for a system as illustrated in FIGS. 1 and 2; FIG. 4 is a
frequency spectrum plot comparing components of a drive signal in
filtered and unfiltered states as observed with a motor winding
connected as the test object;
[0014] FIG. 5 is a frequency spectrum plot showing components of a
discharge signal detected in the drive signal of FIG. 4 when
applied to a motor winding;
[0015] FIG. 6 is a frequency spectrum plot comparing components of
a second drive signal in filtered and unfiltered states as observed
with a motor winding connected as the test object;
[0016] FIG. 7 is a frequency spectrum plot showing components of a
discharge signal detected in the drive signal of FIG. 6 when
applied to the motor winding;
[0017] FIG. 8 illustrates a process flow diagram for evaluating
insulation defects in a variable speed motor; and
[0018] FIGS. 9 and 10 are time domain plots showing phase-resolved
detection of discharge inception.
DETAILED DESCRIPTION
[0019] Systems for off line and on line testing are illustrated in
FIGS. 1 and 2. In FIG. 1, the insulated windings of a variable
speed motor are driven by the output of a high voltage pulse
generator. While the illustrated windings are Delta-connected, the
invention is equally applicable to Wye-connected windings. In FIG.
2, the windings are driven by a drive signal generator providing a
three-phase sinusoidal drive signal. The sinusoidal signal may be
provided as a series of square pulses that approximate a sinusoidal
waveform. In either case, when the peak voltage of the signal
applied to the windings exceeds the corona inception voltage of the
windings, an electrical discharge will occur as described
above.
[0020] In both systems, at the drive input of the motor, a signal S
at the drive input is coupled by a high frequency current
transformer. A high pass filter receives the signal coupled by the
current transformer. The high pass filter preferably has eight or
more poles. The high pass filter and the high frequency current
transformer operate together as a circuit that will be referred to
herein as a sensor. The transfer function of the sensor is chosen
in consideration of the characteristics of the drive signal and a
discharge event detection threshold, as discussed below.
[0021] The output of the sensor is received by a detector/analyzer
unit. The detector/analyzer may be an oscilloscope or spectrum
analyzer, or suitable recording devices such as a fast A/D
converter. The function of the detector/analyzer is to produce an
indication of a discharge based on detection of components in the
output of the sensor that result from the discharge. The sensor and
detector/analyzer thus comprise a system for evaluating insulation
defects in a variable speed motor.
[0022] The sensor is fully characterized by a cut-off frequency and
a rejection. This is mapped by the transfer function shown in FIG.
3. As noted above, the magnitude of the drive waveform is typically
on the order of 100 kV and 100 A, with a spectrum that may exceed 5
MHz. In contrast, a discharge signal has a magnitude on the order
of 10-100 micro Amps, and a frequency spectrum generally beginning
in the range of 100 kHz and extending up to approximately 200 MHz.
Thus, not only does the drive signal spectrum overlap a significant
portion of the discharge signal spectrum, but it does so by 50 dB
or more. Accordingly, to permit detection of discharge signal
components, the sensor must reject components at the high end of
the drive signal frequency spectrum, while passing components of
the discharge signal that lie outside the drive signal spectrum. In
other words, the sensor has a transfer function characterized by a
cutoff at a frequency within an overlap of the frequency spectrum
of the drive signal and a frequency spectrum of a discharge event,
and having a rejection for reducing components of said drive signal
to below a predetermined detection threshold for a discharge event.
An exemplary transfer function of such a sensor is shown in FIG. 3.
As noted in FIG. 3, the transfer function pertains to a combination
of a high frequency current transformer and high pass filter, as
illustrated in FIGS. 1 and 2. The sensor has a cutoff frequency of
15 MHz and provides rejection of 70 dBm at approximately 8 MHz.
This transfer function effectively allows passage of some discharge
signal components while suppressing drive signal components in a
band that overlaps the discharge signal components. In practice, it
has been found that a sensor having a cutoff in the range of
approximately 5 MHz to 20 MHz, and a rejection of 60 dB within one
decade of the cutoff frequency provides an acceptable output
signal. The sensor employed should have the sharp cut-off frequency
shown in FIG. for optimal performance. Lesser cut-off capability
may also work, but makes the final data analysis more difficult and
sometimes less sensitive.
[0023] Alternatively, one may tune the drive waveform to lessen the
high-frequency components that overlap the discharge signals,
thereby opening up a larger viewing window, enabling easier or more
sensitive detection. In practice, this is often difficult because
most commercial drives have a non-user-adjustable waveform output
that is pre-set by hardware and firmware algorithms.
[0024] The effects of sensors applied for testing of various
commercially available motors are illustrated in FIGS. 4 and 5, and
FIGS. 6 and 7. FIG. 4 shows the frequency spectrum of a variable
speed drive signal such as would be applied to the coil of a
General Electric GEB13 motor. The unfiltered drive signal shows
harmonic content up to 20 with strong signal contributions in the
range from 1 to 5 MHz. FIG. 4 further shows the frequency spectrum
of the drive signal after being filtered through a sensor with a 17
MHz cut-off frequency after discharge inception.
[0025] FIG. 5 shows the filtered drive signal applied to the motor
coil with a voltage below the discharge threshold, and the filtered
signal obtained when the drive waveform of FIG. 4 is applied to the
drive the motor coil with a voltage above the discharge threshold.
The discharge signal components are easily discernable in the
latter signal.
[0026] FIG. 6 shows filtered and unfiltered frequency spectra of
the signal as would be applied at the drive input of a General
Electric 30 horsepower, 460 Volt, random wound motor driven with a
variable speed drive signal. FIG. 7 shows the filtered signal
applied to the motor at voltages below and above the discharge
threshold. Again it is readily seen that the discharge components
are clearly discernable in the filtered signal above the discharge
threshold.
[0027] While the embodiments described above employ high pass
filters in the sensor, it is noted that in some situations it may
be useful to use band-pass or band-reject filters, or combinations
thereof. Filters having eight or more poles are preferably
employed.
[0028] The use of filters as described above may be applied in
embodiments of the invention to evaluate insulation defects in a
variable speed motor. A process in accordance with such embodiments
of the invention is illustrated in FIG. 8. Initially, a frequency
spectrum of the drive signal applied to a variable speed motor is
determined 100. The signal may be a single pulse, or an
approximated sinusoidal waveform. Where a waveform is applied, the
particular waveform depends on the particular type of motor to
which it is applied, and therefore the frequency spectrum that is
encountered in a particular case depends on the motor being tested.
Determining the frequency spectrum may be done manually using a
spectrum analyzer. It is noted that the signal may be tuned to
minimize the high frequency harmonics. After the frequency spectrum
has been determined, a sensor is selected 110. Selection of the
sensor involves determination of the transfer function that is
necessary to suppress high frequency components of the drive signal
to a level below the discharge signal detection threshold, as
described above. Once a sensor is selected, the signal at the drive
signal input of the motor winding is applied 120 to the sensor. As
noted above, the signal applied to the sensor includes both drive
signal components and discharge signal components. The drive signal
voltage is then varied 130, and a lowest voltage associated with
inception of a discharge is determined 140. In other words, as the
voltage is varied, for example by beginning at a low voltage and
raising the voltage, the filter output is monitored for discharge
signals, so that a lowest drive signal voltage producing a
discharge can be identified, for example, the voltage at which the
first discharge is produced as the voltage is raised. This lowest
voltage is the corona inception voltage. It may be further
desirable to determine the magnitude of the discharge at the corona
inception voltage, which may be accomplished by observation of the
discharge components in the output of the filter at the corona
inception voltage, or to determine the drive signal polarity at
inception.
[0029] In alternative embodiments of the invention, a fast rise
time wave may be applied to an insulated coil, and the voltage of
the wave may be varied to detect a corona inception voltage. An
appropriate wave generator can be generated by a surge tester such
as Baker Instruments model D12000. Such alternative embodiments are
suitable for off line evaluation of motor coils such as would be
practiced for quality assurance and control testing.
[0030] The above process steps may be manually performed, for
example by observing the frequency spectrum of a drive signal on a
spectrum analyzer, manually selecting a filter circuit, connecting
the filter circuit so that the signal is applied to the filter,
manually varying the voltage of the drive signal by means of
controls of the drive signal pulse generator, and observing the
filter output on an oscilloscope to detect discharge. However,
automated embodiments of the process are also contemplated, for
example, a process enabled by a control system that controls drive
signal voltage, monitors drive signal spectrum and filter output
spectrum, and selects and applies a filter having an appropriate
transfer function.
[0031] FIGS. 9 and 10 illustrate exemplary time-based data showing
a drive waveform and discharge signals produced in response to the
drive waveform, which occur at drive waveform maxima. FIG. 9 shows
discharge activity in response to a drive waveform that is just
above discharge inception voltage, while FIG. 10 shows discharge
activity in response to a drive waveform that is well above the
discharge inception voltage. Comparison of the FIGS. shows the
greater discharge activity is produced at higher drive voltages,
and that discharge is statistically most likely to occur at drive
signal maxima, although the discharge amplitude may vary with
signal polarity.
[0032] The signals from the filters can be used to enable simple QA
or QC purposes, depending upon the manufacturing needs. On-line or
off-line testing of units in the field can also be enabled by this
technology.
[0033] While the embodiments described above are the embodiments
presently preferred by the inventors, a variety of alternative
implementations may be employed without departing from the scope of
the claimed invention.
* * * * *